Based on @MarkSalamon suggested link: http://blog.softartisans.com/2013/12/31/kb-creating-dynamic-links-with-mergefields-in-microsoft-word/ , it failed in my case too.
After debugging the opensagres library, it seems that instead of inserting a merge field inside the Hyperlink, the library is expecting there just a simple Freemarker placeholder. So it's enough to create the hyperlink from Word, and at the url definition part, you can specify there: ${url} .
That's it, the library is going to detect that there is a Freemarker syntax and replace the url with back-end data. So you don't have to do the trick with ALT+F9 and replace with merge field codes.
In my case this worked with opensagres version: 2.0.2